Choosing to vote Democratic is not the same as chattel slavery. Any black can choose to vote Republican if they choose (we do, after all, have secret ballots in this country). A chattel slave could not just quit working for his master and leave the plantation.
Even poorly used free will is much better than knowing that your entire life and that of your children and grandchildren rested on the whims, morals, and fate of your master.
Indentured servants, while they may have been pushed harder than slaves during their service, had a specific date in which they would be freed, and often given property. Additionally, the servant often voluntarily chose to become an indentured servant. Slaves didn't.
Slaves might be freed if their master was kind. If the master decided to free the slave, often that freedom came after the master had passed. Slaves could be separated from their families. Spouses from each other. Children from either or both parents and their siblings. Furthermore, the child of a slave was also a slave.